It is known that gases under pressure are stored and marketed in elongated metal gas-pressure tanks, usually composed of steel having a closed bottom and a mouth to which a pressure reducer or other valve arrangement may be affixed for the dispensing of the gas.
In many cases, the stored gas is required to be maintained at high purity. This is particularly the case for gases which are used in scintillation, chromatographic and other analytical processes, for gases which are utilized to maintain a predetermined atmosphere for analytical and reaction purposes, and for gases which are to constitute standards, e.g. for analytical purposes, as well as for gases which are required in a high purity state for reactions involving the gases themselves.
When such gases are exposed to the steel surfaces forming the inner wall of such a tank, however, there is the possibility of contamination of the gases by the materials of which the wall may be constituted, thereby reducing the purity of the gas or rendering it unfit for particular purposes.
It has been proposed, therefore, to line interiors of such tanks with substances which themselves do not contaminate the gas and which serve as a barrier to contamination of the gas by materials from the underlying wall structure.
However, the very nature of the tanks, namely, their long dimensions, the narrow mouth providing the only access and the closed bottom, create coating problems for conventional coating techniques. Furthermore, many conventional coating techniques which might be useful for applying, for example, synthetic resin coatings, cannot be used effectively to apply highly adherent relatively thin coatings of more esoteric materials, for example, the nitrides, borides and carbides of refractory metals such as tungsten, titanium, tantalum and zirconium.
In the above-mentioned application and patents, there is disclosed a method and apparatus for applying metals and their compounds to a variety of substrates utilizing the repeated striking of an arc to generate vapor from at least one of the electrodes so that the vapor, with or without reaction with other materials, is caused to deposit upon a substrate which is provided together with the arc-striking system in an evacuated vessel or chamber.
For this system, large-area coating and the internal coating of pipes could make use of the fact that, after being struck and before being quenched by the next contact between the electrodes, the arc may travel for some distance along the electrode to permit evaporation over a substantial length of the electrode.
However, that approach as described in the aforementioned patent and application is not directly applicable to the internal coating of gas-pressure bottles and tanks of the type described because the introduction of the gas-pressure tank into an evacuated vessel is not practical.